Orthodontic treatment often involves attaching an appliance to the tooth or attaching one appliance to another orthodontic appliance previously attached to the tooth. Forces applied to the appliance(s) are then transferred to, and thus move, the tooth. As such, orthodontic appliances represent a principal component of corrective orthodontic treatment devoted to improving a patient's dentition. Orthodontic appliances may include brackets, archwires, hooks, bands, and other devices.
Using the orthodontic bracket as an example, an orthodontist may affix orthodontic brackets to the patient's teeth with an adhesive and then engage an archwire into a slot of each bracket. The archwire exerts flexural and/or torsional stresses on the orthodontic brackets to create restorative forces, including rotation, tipping, extrusion, intrusion, translation, and/or torque forces, tending to bring the teeth toward a desired, aesthetic position. Traditional ligatures, such as small elastomeric O-rings or fine metal wires, may be employed to retain the archwire within each bracket slot. Due to difficulties encountered in applying an individual ligature to each bracket, self-ligating orthodontic brackets have been developed that eliminate the need for ligatures by relying on a movable portion or member, such as a latch, clip, or slide, for retaining the archwire within the bracket slot.
In a typical sequence of orthodontic treatment, a small diameter round metallic archwire is used for preliminary tooth movement, followed by the use of rectangular metallic archwires at the later stages of treatment. The final stage may involve the use of an archwire of rectangular cross-section which fills the slot in the bracket. For example, a small (e.g., 0.014 inch diameter) round archwire may be used initially and a rectangular cross-sectioned (e.g., 0.021 inch by 0.025 inch) archwire may be introduced when torque is required to precisely orient the teeth, usually at or near the end of treatment. Since its rectangular shape renders it non-rotatable with respect to each bracket, the archwire imposes torquing or uprighting forces on the teeth. As a result, the rectangular wire may be slightly twisted between adjacent teeth. Other archwires of different sizes may be introduced during intermediate stages of treatment.
Where malocclusion is severe, it is generally not practical to commence treatment with large cross-sectional archwires for several reasons. Most significantly, the bracket slots are generally not in alignment with each other so that the archwire must be substantially twisted or deflected at the commencement of treatment. Since large cross-sectioned archwires undergo permanent deformation more easily than smaller wires, initial twisting or deflection may render them nearly useless at the beginning of treatment. Large wires may also exert unpredictably large forces during the initial phase of treatment, which may be extremely painful for the patient. For at least this reason, it is often necessary to use smaller archwires initially and then replace the small archwires with larger cross-sectional archwires as treatment progresses. For the patient, this means frequent appointments and significant time spent in “the chair.” For the clinician, this means increased costs and reduced treatment capacity.
Early stage wires are typically made of shape memory alloys (SMAs) with superelastic properties. SMAs undergo a reversible crystalline phase transformation from a martensitic phase to an austenitic phase when heated through a particular temperature range. Generally, martensite is soft and ductile while austenite is rigid and elastic. Because these two phases provide individually unique mechanical properties, the temperature of the alloy during use dictates the mechanical properties of the alloy according to the proportions of martensite and austenite. Therefore, the phases present when an orthodontic appliance is used at the temperature of the human body will determine the mechanical properties of the appliance.
In this regard, the temperature at which the martensite-to-austenite phase change starts is generally denoted as As, referred to as the “austenitic start temperature,” and the temperature at which the phase changes finishes upon heating is denoted as Af, referred to as the “austenitic finish temperature.” Above Af, the stable phase in the alloy is the austenitic phase. During cooling, the temperature at which the phase change from austenite to martensite starts is denoted as Ms, referred to as the “martensitic start temperature,” and the temperature at which the phase change finishes is denoted as Mf, referred to as the “martensitic finish temperature.” The reversible phase transformation permits SMAs to be deformed at one temperature and then heated to an elevated temperature where the SMA recovers all or nearly all of its predeformed or original shape. NiTi-based alloys are well-known shape memory alloys and are alloys of nickel (Ni) and titanium (Ti). For example, one type of NiTi-based alloy is nitinol, which is an alloy of approximately 50/50 nickel to titanium.
In addition, SMAs often exhibit superelastic characteristics. Superelasticity results from stress-induced phase transformation of austenite to martensite at or slightly above Af. Reversion to austenite occurs as the strain is reduced or removed. The stress-strain behavior of superelastic materials used in orthodontics takes full advantage of this phase change and often allows recovery of up to 6% strain, well beyond conventional stainless steels.
Current manufacturing technology focuses on a pre-defined process that controls the composition of the alloy, the thermal treatment of the alloy, and the stresses induced into the alloy during manufacturing of the orthodontic appliance. Collectively these parameters establish the transformation properties, i.e., the shape of the transformation curve defined by the temperatures As, Ms, Af, and Mf, of the SMA.
While orthodontic appliances have been generally successful, manufacturers of orthodontic appliances continually strive to improve the performance of their appliances. In this regard, there remains a need for superelastic and/or shape memory orthodontic appliances that provide improved performance during orthodontic treatment.